Subtracting value from the result of a function

Question

I'm using the module 'ngCart' to act as a shopping cart for a website and I am trying to set up an additional way to implement a voucher system my app.

so far, my method looks like this:

    ngCart.applyFiveVoucher = function(){
        if(ngCart.setFiveVoucher) {
            this.totalCost() -= 5
        }

but is returning:

ReferenceError: Invalid left-hand side in assignment

I understand that you cannot alter the return of a function but I am unable to edit the method within the module as these changes will not be passed on if other people try and use my repo. Any suggestions?

Answer 1

You cannot assign to a function result.

ngCart.applyFiveVoucher = function(){ if(ngCart.setFiveVoucher) { return this.totalCost() - 5; }